As it stands now it looks like the Senate stays Republican, but there could be shake-ups. The Senate race in Georgia is huge since there are two seats in play there, but that won't be determined until January. If the Dems take those seats the Senate is tied, and Harris as VP is the tiebreaker. That opens up for them to make some significant changes. I expect them to get rid of a lot of the Republican ratfeckery that has plagued American politics in the last few years. Gerrymandering, filibuster and so on. If they don't win the Senate McConnell is just going to keep acting like a complete cnut and Dems won't be able to do anything substantial in that regard.